Nuclear spin relaxation in isotropic and anisotropic media

A unified, self-consistent description of the microscopic interactions that influence dynamics in spin-space for an ensemble of spin-1/2 particles, as well as the real-space effects of the macroscopic (classical) global rotational diffusion determined both by molecular shape and the nature of the solvent is studied. The combined effects of the two on the spin-relaxation rates measured by NMR spectroscopists is also studied. The measurement of amide ¹⁵N spin-lattice (R₁), spin spin (R₂) and the steady-state NOE with the attached hydrogen has become routine for practitioners of biomolecular NMR spectroscopy. These rates can be used to determine the underlying spectral density functions and to interpret them using the Lipari-Szabo formalism that relies on the separation of the global rotational diffusion and local dynamics on single or multiple timescales. All spectral density functions in this review have been derived assuming that the spin-system under consideration is rigidly attached to the biomolecule and that the only motion results from the overall rotational diffusion.